1. Field of the Invention
The present invention relates to a structure of a semiconductor device having a semiconductor circuit comprised of semiconductor elements such as insulated gate transistors and a method for manufacturing the same. More particularly, the invention relates to a structure of a semiconductor device having a semiconductor circuit comprised of semiconductor elements having an LDD structure formed of organic resin and a method for manufacturing the same. Semiconductor devices according to the invention include not only elements such as thin film transistors (TFTs) and MOS transistors but also displays having a semiconductor circuit and electro-optical devices such as image sensors formed by such insulated gate transistors. In addition, semiconductor devices according to the invention include electronic equipments loaded with such displays and electro-optical devices.
2. Description of the Related Art
TFTs have been conventionally used as switching elements of active matrix liquid crystal displays (hereinafter abbreviated to read “AMLCD”). The market is currently dominated by products having circuits formed by TFTs utilizing an amorphous silicon film as an active layer. Particularly, a widely used TFT structure is the reverse staggered structure, which allows simple manufacturing steps.
However, the accelerating trend toward AMLCDs with higher performance in recent years has resulted in more severe requirements on the operational performance (especially, operating speed) of TFTs. It has therefore become difficult to provide elements having sufficient performance utilizing TFTs comprising amorphous silicon films because of their operating speed.
Under such circumstances, TFTs utilizing polycrystalline silicon films (polysilicon films) have come into focus in place of amorphous silicon films, which has significantly accelerated the development of TFTs utilizing a polycrystalline silicon film as an active layer. Presently, some products have already been introduced.
Many reports have already been made on structures of reverse staggered TFTs utilizing a polycrystalline silicon film as an active layer. However, conventional reverse staggered structures have had various problems.
First, since an active layer as a whole employed in such structures is as very thin as about 50 nm, impact ionization occurs at the junction between a channel formation region and a drain region, which results in significant deteriorating phenomena such as the implantation of hot carriers. This necessitates the formation of an LDD region (light doped drain region).
It is expected that at least eight masks are required (for processes up to the formation of source and drain electrodes) to form such an LDD region in a conventional reverse staggered TFT structure.
As described above, in a conventional reverse staggered TFT structure, an LDD region must be formed in a horizontal plane on both sides or one side of a channel formation region, which makes it very difficult to form LDD regions with reproducibility.
It is an object of the invention to provide a technique for manufacturing semiconductor devices with high mass-productivity, reliability and reproducibility through very simple manufacturing steps.